Electronic structure and magnetism of PrNixPt1−x compounds

We have studied influence of the Pt–Ni substitution on the crystal structure and magnetic behavior of the PrNi_xPt_1−x compounds. Polycrystalline samples with x = 1, 0.9, 0.75, 0 were prepared and characterized by X-ray powder diffraction (XRPD) and scanning electron microscopy (SEM). The analysis of XRPD data confirmed that the orthorhombic CrB-type structure owned by the parent binary compounds remains conserved through the entire series. The samples were subsequently investigated by specific heat (C_p), magnetization (M) and ac susceptibility measurements in the temperature range 2–350 K and in magnetic fields up to 9 T. All compounds were found to order ferromagnetically. The T_C values monotonously increase with increasing Ni content. To inspect the crystal-field (CEF) effects and magnetocaloric properties specific-heat data were analyzed in detail and the magnetic contribution to the specific heat together with the magnetic entropy have been determined. The results of first principles electronic structure calculations of the PrNi and PrPt confirmed that besides the stable Pr magnetic moments due to localized 4f-electrons only a very small magnetic moments of at most 0.2μ_B is induced at the Ni (Pt) site due to the polarized 3d-electron states (5d-electron states) hybridizing with the Pr 5d-electron states, i.e. the Ni (Pt) moment plays only minor role in the total balance of the magnetic moments in these compounds.     

Magnetic properties of Ce3−xGdxCo11B4 borides

The structure and magnetic properties of Ce_3−xGd_xCo₁₁B₄ borides have been studied by X-ray powder diffraction (XRPD), magnetization and differential scanning calorimetry (DSC) measurements. X-ray analysis reveals that the compounds crystallize in the hexagonal Ce₃Co₁₁B₄-type structure with P6/mmm space group. The substitution of Gd for Ce leads to an increase of the unit-cell parameter a and the unit-cell volume V, while the unit-cell parameter c decreases linearly. Magnetic measurements indicate that all samples are ordered magnetically below the Curie temperature. The Curie temperatures increase as Ce is substituted by Gd. The saturation magnetization at 4 K decreases upon the Gd substitution up to x = 1, and then increases.     

Magnetic properties and spin-reorientation transition in DyFe10−xNixSi2 compounds

The magnetic properties of DyFe_10−xNi_xSi₂ compounds with x = 0, 1, 2, 3, 4, 6, 9 and 10 have been investigated by means of X-ray diffraction and magnetic measurements. Substitution of Ni for Fe leads to a decrease in the lattice constants a, c and the unit-cell volume V. The Curie temperature reaches a maximum of 590 K at x = 2, then decreases strongly for x ≥ 2. The spin reorientations are observed for the compounds with x = 0, 1, 2 and 3. The spin reorientation temperature decreases strongly from 255 to 60 K as the Ni content is increased from x = 0 to 3. Below the spin reorientation temperature, the compounds exhibit ferrimagnetic ordering. For the Ni-rich compounds with x = 9 and 10, the magnetization of the Dy sublattice decreases strongly since the magnetization of the Dy sublattice is strongly affected by the molecular field produced by the 3d sublattice.     

Magnetic properties of CeMn2−xCoxGe2

The structure and magnetic properties of CeMn_2−xCo_xGe₂ (0.0≤x≤1.0) were studied by X-ray powder diffraction and magnetization measurements. All compounds crystallize in the ThCr₂Si₂-type structure with space group I4/mmm. Substitution of Co for Mn leads to a linear decrease in the lattice constants and the unit cell volume. Increasing substitution of Co for Mn shows a depression of ferromagnetic ordering.     

Structural, magnetic and electrical transport properties for a series of La1−xSrxFe1−xMnxO3 (0.3 ≤ x ≤ 0.7) compounds

The structural, electrical transport and magnetic properties have been studied for compounds: La_1−xSr_xFe_1−xMn_xO₃ (0.3 ≤ x ≤ 0.7). The lattice parameter, a, first decreases with x, and followed by an increase when Sr²⁺ and Mn⁴⁺ was continuously doped. The cell parameters, b and c, slightly decrease with coupled substitution of Sr²⁺ for La³⁺ and Mn⁴⁺ for Fe³⁺. In the paramagnetic temperature range, formation of magnetic clusters is suggested; the sizes of clusters decrease with x up to 0.5, following that they increase sharply with continuing doping. The electrical behaviors of all specimens demonstrate insulators and the electrical resistivity increases with content of Mn⁴⁺ and Sr²⁺ ions doped. A variable range hopping model is suitable to describe electrical transport process for the compounds at low temperature. At high temperature the electrical transport process can be described by bipolaron model for all compounds.     

Effect of Pr and Co substitution on magnetic properties and magnetic entropy changes in LaFe13−xSix compounds

Effect of Pr and Co substitution on magnetic properties and magnetic entropy changes in the cubic NaZn₁₃-type compound LaFe_11.2Si_1.8 has been experimentally investigated. Replacing 30 at.% La with Pr leads to a decrease of Curie temperature from 216 to 203 K, and drives the magnetic transition from second-order to first-order. As a result, magnetic entropy change, under a field change of 0–5 T, increases from 13.7 to 19.4 J/kg K. Substitution of Co for Fe in La₀₇Pr_0.3Fe_11.2Si_1.8 can adjust T_C to around room temperature. A magnetic entropy change of 9.3 J/kg K at T_C = 290 K for a field change from 0 to 5 T is obtained in La₀₇Pr_0.3Fe_10.4Co_0.8Si_1.8. A reversible variation of magnetization with temperature and magnetic field is observed in the present compound, which is highly desired by the magnetic refrigeration application.     

Crystallographic site of Mn in the icosahedral cluster of LaCo13−xMnx compounds

Studies on the structure and the crystallographic site of Mn in LaCo_13−xMn_x compounds were carried out by using X-ray diffraction and X-ray absorption fine structure (XAFS) of the Mn K-edge. These compounds with x≤3.0 adopt a NaZn₁₃-type structure consisting of icosahedral clusters. The lattice constant increases with the Mn concentration. The calculated XAFS curves of the center and the corner sites in the icosahedral clusters for the Mn K-edge are obtained by using the program . The fitting result for the corner site agrees much better with the observed XAFS spectrum than that for the center site. Therefore, the Mn site is determined to be the corner Co site in the icosahedral clusters of all the compounds. In comparison with the crystallographic parameters of LaCo₁₃, the icosahedral clusters composed of Mn atoms expand and the crystallographic Mn site is slightly more close to the La atom.     

Magnetic properties of Pr1−xGdxMn2Ge2 compounds

The structure and magnetic properties of the Pr_1−xGd_xMn₂Ge₂ (0.0≤x≤1.0) compounds have been investigated by means of X-ray diffraction (XRD), differential scanning calorimetry (DSC) techniques and AC magnetic susceptibility measurements. All compounds crystallize in the ThCr₂Si₂-type structure with the space group I4/mmm. The lattice constants and the unit cell volume obey Vegard’s law. Samples in this alloy system exhibit a crossover from ferromagnetic ordering for PrMn₂Ge₂ to antiferromagnetic ordering for GdMn₂Ge₂ as a function of Gd concentration x. At low temperatures, the rare earth sublattice also orders and reconfigures the ordering in the Mn sublattice. The results are summarized in the x–T magnetic phase diagram.     

Magnetocrystalline anisotropy of Nd3(Fe1−xCox)27.7Ti1.3Ny compounds

The effect of Co substitution for Fe in Nd₃(Fe_1−xCo_x)_27.7Ti_1.3N_y (0 ≤ x ≤ 0.4) compounds on the magnetocrystalline anisotropy has been investigated. The anisotropy constants K's and the anisotropy field H_A have been deduced from the magnetization curves measured on magnetically aligned powder (4–7 μm) samples. The obtained results show that at RT the anisotropy is uniaxial and H_A (about 10 T) does not change substantially upon the substitution. At 5 K the results for K's give evidence for the presence of easy-cone-type anisotropy. The cone angle as well as the anisotropy field decrease upon the substitution from 21.6° to 11.8° and from 22.8 to 18.6 T, respectively.     

Magnetic properties of (Fe)1−x–(Al2O3)x and (Fe50Ni50)1−x–(Al2O3)x nanocomposite magnetic media synthesized using gel like Al2O3 matrix

Composites with ferromagnetic nanoparticles, Fe and Fe₅₀Ni₅₀, dispersed in Al₂O₃ have been synthesized by a solution phase technique. The structure and magnetic properties of these composites with varying fractions of Al₂O₃ have been investigated. Both Fe and Fe₅₀Ni₅₀ nanoparticles are amorphous in the as-prepared state and become crystalline on heat treating with near equilibrium lattice parameters of 0.287 nm and 0.358 nm respectively. The interparticle distance increases with increasing Al₂O₃ from 0 wt.% to 20 wt.%. The size of Fe nanoparticles is 40 nm while the Fe₅₀Ni₅₀ nanoparticles are 20 nm in size. The Fe and Fe₅₀Ni₅₀ nanoparticles dispersed composites are found to be ferromagnetic at room temperature both in the as-prepared and heat treated conditions with clear coercive fields of 5.5–35 × 10³ A m⁻¹. The saturation magnetization increases by orders of magnitude on heat treatment, for e.g. from <1.0 emu g⁻¹ to 143.4 emu g⁻¹ for Fe–15 wt.% Al₂O₃ and 95.6 emu g⁻¹ for Fe₅₀Ni₅₀–15 wt.% Al₂O₃. The Fe-composites exhibit a Curie transition at 1000 K while the Fe₅₀Ni₅₀ composites exhibit a transition at 880 K, both temperatures close to bulk values.     

Structure and magnetic properties of TbFe10.5−x MnxMo1.5 compounds

The effects of Mn partial substitution for Fe in TbFe_10.5Mo_1.5 on the structure and magnetic properties were investigated. TbFe_10.5−xMn_xMo_1.5 samples (x = 1.5, 2.0, 3.0, 4.0, 5.0) were prepared by means of arc-melting and subsequent vacuum annealing. The structure and magnetic properties of TbFe_10.5−xMn_xMo_1.5 compounds were investigated by X-ray powder diffraction and magnetic properties measurements. The following conclusions were obtained: all the TbFe_10.5−xMn_xMo_1.5 compounds studied crystallize in the ThMn₁₂-type structure; the unit-cell volume increases monotonically with increasing Mn concentration; a compensation temperature was observed in the magnetization-temperature curve of TbFe_7.5Mn₃Mo_1.5 compounds. With increasing Mn concentration, the saturation magnetization at 4.4 K decreases to zero, and then increases again, the magnetic moments of the transition-metal sublattice of TbFe_10.5−xMn_xMo_1.5 compounds decrease monotonically.     

Correlation between the synthesis conditions and the compositional and magnetic properties of Co2(Cr1−xFex)Al Heusler alloys

In this study we give evidence for the strong dependence of the compositional and magnetic properties on the synthesis conditions of polycrystalline Co₂(Cr_1−xFe_x)Al Heusler alloys (0 ≤ x ≤ 1) by comparing the properties of as-grown and annealed compounds. Strong chemical inhomogeneities are found at the micrometric level depending on the compound and the synthesis method. Moreover, we find that the Co content is homogeneous at the micrometric level in all the studied samples in sharp contrast with significant inhomogeneous distribution of (Fe/Cr) and Al at the micrometric level, especially for Cr-rich compounds (x ≤ 0.4). We have found that the magnetic properties (the Curie temperature and the saturation magnetization) are strongly depressed in the annealed compounds with respect to the corresponding as-grown compounds. For the as-grown compounds the saturation magnetization is close to the theoretically predicted one for x ≥ 0.7 whereas it is lower than the theoretically predicted one for x ≤ 0.4, which correlates with the observed chemical inhomogeneity.     

Structural and magnetic properties of Sm2Fe17−xCrxC2 nanocrystalline carbides with 0≤x≤2

The crystallographic and the Curie temperature of the Sm₂Fe_17−xCr_xC₂ (x=0.5, 1, 1.5 and 2) carbides have been extensively studied. X-ray diffraction studies have shown that all these alloys are approximately single phases corresponding to the Th₂Zn₁₇ type rhombohedral structure with a small amount of -Fe. The amount of this residual -Fe phase decreases with increasing the Cr atomic content. It decreases from 1 wt% for x=0.5 to 0.4 wt.% for x=2. The lattice parameter c increases as a function of the Cr atomic content x from x=0 to x=1.5 and then decreases. This is due to the Cr atoms which prefer to substitute the Fe atoms in the 6c sites located along the c-axis. The lattice parameter a and the unit-cell volume decrease in all substitution ranges. The insertion of the C atoms leads essentially to an increase of the distances between the 9d and 18h sites and the 9d–18f sites. The Curie temperature reaches a maximum value of 583 K for x=1.5 and then decreases to 551 K for x=2. The enhancement of the T_c for lower Cr contents is due to a lowering of the hybridization of the iron atoms with their neighbors, the magnetovolume effect and the reduction of antiferromagnetic interactions. However, the decrease in T_c for higher Cr content is due to the reduction in the number of Fe–Fe pairs due to the magnetic dilution effect. For given interatomic distances, the exchange coupling of the Cr–Cr atoms is not of antiferromagnetic type and the exchange integral of the Cr–Cr pair is higher than that of the Fe–Fe pair.     

Thermoelectric performance of Mg2−xCaxSi compounds

Thermoelectric materials Mg_2−xCa_xSi (x = 0, 0.01, 0.03, 0.05, 0.07, 0.1) compounds have been prepared by vacuum melting followed by hot-pressing. Effects of the substitution of Ca for Mg on phase structures and the thermoelectric properties of the hot-pressed compounds were investigated. It was found that the alloying of Ca in Mg₂Si based compounds increases the electrical conductivity and decreases the Seebeck coefficient of the compounds, due to the electronegativity difference between Ca and Mg. The dimensionless figures of merit of Mg₂Si and Mg_1.99Ca_0.01Si reach, respectively, 0.41 and 0.34 at 660 K.     

The effects of the combined substitution of Y and Ga on the crystallographic structure of Nd2−xYxFe17−yGay intermetallic compounds

The effects of the combined substitution of Y and Ga on the crystallographic structure of Nd_2−xY_xFe_17−yGa_y compounds with x = 0, 0.5, 1.0, 1.5 and y = 0, 1, 2, 3 have been investigated using X-ray and neutron powder diffractions. Rietveld refinements of the diffraction data indicate that all the samples crystallize in the rhombohedral Th₂Zn₁₇-type structure with only small amounts of alpha iron. It is found that the addition of Ga atoms lessens the decreasing rates of the a-axis and unit cell volume V on the Y content but almost does not affect the decreasing rates of the c-axis. However, the substitution of Y has a positive effect on the increasing rates of the a-axis and unit cell volume V on the Ga content but has a very slight effect on the increasing rate of the c-axis. The c/a ratio of Nd_2−xY_xFe_17−yGa_y as a function of Ga content exhibits a different increase for different Y content owe to the combined effects of Y and Ga on the crystallographic structure. The substitution of Y is found to have little effect on the site occupancy of Ga in Nd_2−xY_xFe_17−yGa_y. The combined effects of Y and Ga on the bond lengths and ASBL of Nd_2−xY_xFe_17−yGa_y indicate that more bonds detrimental to ferromagnetic exchange can be modulated into the desirable ferromagnetic exchange distance range through suitable combined substitution, which provides a valuable way to improve the magnetic properties of rare earth-transition intermetallic compounds.     

Glass-forming ability and magnetic properties of mechanically solid-state reacted Co100−xTix alloy powders

A high-energy ball milling technique using the mechanical alloying method has been employed for fabrication of glassy Co_100−xTi_x (25≤x≤67) alloy powders at room temperature. The fabricated glassy alloys in the Co-rich (33≥x) side exhibit good soft magnetic properties. The binary glassy alloys for which the glass transition temperatures (T_g) have rather high temperatures (above 800 K), show large supercooled liquid regions before crystallization (ΔT_x larger than 50 K). The reduced glass transition temperature (ratio between T_g and liquidus temperatures, T_l (T_g/T_l)) was found to be larger than 0.56. We have also performed post-annealing experiments on the mechanically deformed Co/Ti multilayered composite powders. The results show that annealing of the powders at 710 K leads to the formation of a glassy phase (thermally enhanced glass formation reaction), of which the heat of formation was measured directly. The similarity in the crystallization and magnetization behaviors between the two classes of as-annealed and as-mechanically alloyed glassy powders implies the formation of the same glass state.     

Evolution of magnetic hardness in the HfFe6Ge6-type RMn6Sn6−xGax and RMn6Sn6−xInx compounds (R=Tb, Dy; 0.2≤x≤1.4)

The HfFe₆Ge₆-type RMn₆Sn_6−xX_x′ solid solutions (R=Tb, Dy, X′=Ga, In; x≤1.4) have been studied by powder magnetization measurements. All the series are characterized by ferrimagnetic ordering and by a decrease in Curie temperatures with the substitution (ΔT_c/Δx≈−39 K for X′=Ga and ΔT_c/Δx≈−75 K for X′=In). The RMn₆Sn_6−xGa_x systems are characterized by a strong decrease in the spin reorientation temperature with substitution (ΔT_t/Δx≈−191 K and −78 K for R=Tb and Dy, respectively) while this transition almost does not change in systems containing indium. The coercive fields drastically decrease with the substitution in the TbMn₆Sn_6−xGa_x system while the substitution of In for Sn has a weaker effect. The coercive fields of the Dy compounds do not vary greatly with the substitution in both series. The behaviour of the TbMn₆Sn_6−xGa_x is compared with the evolutions observed in the TmMn₆Sn_6−xGa_x series. This comparison strongly suggests that the replacement of Sn by Ga changes the sign of the A₀² crystal field parameter.     

Magnetic phase transitions in Nd1−xYxMn2Ge2 (0 ≤ x ≤ 0.6)

The structure and magnetic properties of Nd_1−xY_xMn₂Ge₂ (0.0≤x≤0.6) were studied by X-ray powder diffraction and magnetization measurements. All compounds crystallize in the ThCr₂Si₂-type structure with space group I4/mmm. Substitution of Y for Mn led to a linear decrease in the lattice constants and the unit cell volume. Increasing substitution of Y for Nd in NdMn₂Ge₂ shows a depression of ferromagnetic ordering and the gradual development of antiferromagnetic ordering.     

Extrinsic and intrinsic magnetic properties of Co1−x Fex Sb3

We report magnetic properties of iron in Co_1−x Fe_x Sb₃ for x in the range 0<x<0.2, since x=0.2 is found to be the limit of solubility of iron in the skutterudite lattice. The magnetic ions diluted in the matrix carry a small magnetic moment reduced to that of the spin-only S=1/2 value of the Fe³⁺ in the low spin d⁵ configuration in presence of a strong crystal field that screens the orbital momentum. The magnetic properties give evidence that a small fraction of iron is spin-frozen in magnetite ferrimagnetic clusters, and antiferromagnetic FeO clusters. Because both types of clusters represent only very minor phases, their detection by the usual analytical means such as X-rays is not possible. The remaining part is diluted in the matrix to form a semimagnetic semiconductor characterized by a Fe–Fe nearest-neighbor exchange interaction J that is antiferromagnetic, with |J|/k_B19.6  K.     

Structural, thermodynamic, and electrochemical properties of TixZr1−x(VNiCrMnCoAl)2 C14 Laves phase alloys

The crystal structure of the monoclinic phase η-Al₁₁Cr₂ of the space group C2/c, a ≈ 1.76 nm, b ≈ 3.05 nm, c ≈ 1.76 nm, β ≈ 90° [L.A. Bendersky, R.S. Roth, J.T. Ramon, D. Shechtman, Metall. Trans. A 22A (1991) 5] has been determined by single-crystal X-ray diffraction. The structure model, refined to a final R value of 0.0441, has the composition of Al_83.8Cr_16.2. a = 1.77348(10) nm, b = 3.04555(17) nm, c = 1.77344(10) nm, monoclinic angle β = 91.0520(12)°. There are 80 (66Al + 14Cr) independent atomic positions in a unit cell, of which all Cr atom sites and 8 Al atom sites have icosahedral coordination. These icosahedra are interconnected forming icosahedral chains along , (1 0 1) icosahedral layer blocks as well as a three-dimensional icosahedral structure.